The invention relates to a transponder antenna, particularly intended for use in RFID (radio frequency identification) transponders. Such devices commonly have an antenna arranged on a carrier substrate or in a housing, as well as an integrated circuit connected to the antenna, said integrated circuit being most commonly present in the form of a semiconductor component. In some cases, a battery is also provided as a power supply.
In principle, either HF or UHF transponders are used, depending on the frequency of data transmission from and to the integrated circuit. When HF transponders are used (so-called loop antennas) particularly flat coil antennas are commonly used.
In the case of such coil antennas, there is always a region in which the conductor path which forms the antenna crosses the turns of the coil formed by said conductor path in order that both ends of the conductor path can be connected to the connectors of the integrated circuit. In order to prevent a short circuit, the segments of the conductor path which cross each other must be electrically insulated from each other. In cases where the antenna is manufactured by forming wire to the desired shape, this can be achieved by using enameled wire, for example.
In the case of antenna structures which are produced by additive methods (e.g. by printing) or by subtractive methods (e.g. by selective etching of a metal layer) on a non-conducting carrier substrate, the antenna is most commonly constructed in two parts.
For example, the coil turns can be arranged on a first side of the carrier substrate along with a bridge which crosses the coil turns, wherein both ends of the bridge are connected in an electrically conducting manner through the carrier substrate, with one end of the conductor path which forms the coil turns on one side, and a contact terminal for the connection to the integrated circuit on the other side. This can be performed, for example, by using vias, or by mechanically connecting the ends, e.g. by stamping. In the case of a mechanical connection (stamping), the non-conducting carrier substrate is punctured, such that the site of the puncture is the site where an electrical contact between the two conductor path structures is created, wherein one said structure is arranged on each side of the carrier substrate.
However, the bridge can also be arranged on the same side of the carrier substrate as the conductor path which forms the coil turns. If the bridge is applied by means of an additive method, it is also important to ensure that the bridge is electrically insulated from the coil turns. This can be achieved by initially overprinting an insulating paste at the point where the bridge will cross the coil turns, then printing a conducting path which forms the bridge.
As an alternative, the bridge can also be produced by using an additional auxiliary substrate. In such a case, a conductor track which will later form the bridge may be attached to the auxiliary substrate by means of, for example, printing with a conductor paste. Next, the auxiliary substrate itself is laid over the carrier substrate in such a manner that the conductor path which forms the bridge crosses the coil turns. Then, one end of the bridge is electrically connected to one end of the conductor path which forms the coil turns, and the other bridge end is electrically connected to a contact terminal which provides a connection to the integrated circuit. This can be done by means of ultrasound welding, for example. In this case as well, a layer of insulation must be inserted between the conductor path on the auxiliary substrate, said conductor path forming the bridge, and the conductor path on the carrier substrate, said conductor path forming the coil turns. This layer of insulation can be produced by printing with an insulating paste, for example. In this case, the insulation layer can either be arranged on the auxiliary substrate or on the carrier substrate before the auxiliary substrate is placed over the carrier substrate. This can be done by printing with an insulating paste, for example.
In the case of the example of RFID transponder antennas provided above, the problem arises that such constructions are not sufficiently flexible for many applications. If such a transponder, the transponder being inserted into a label or a card, is bent significantly, the carrier substrate and the auxiliary substrate are stretched to different degrees. This may result in a disconnection of the bridge, whereby the transponder becomes damaged and therefore unusable. Damage or alteration to the transponder may also occur if its electrical characteristics change as a result of such mechanical loading in such a manner that the insulating bridge is altered in its function of providing capacitance.
Therefore, there is a need for a technical solution for this type of RFID transponders, wherein the solution should improve the mechanical resilience of the antenna with respect to loads that cause bending.